1. Field of the Invention
The present invention relates to a system for controlling the output of an atomic power plant using a boiling-water reactor and more particularly to a control system for preventing the overheating of fuel assembly in the reactor and for securing a safety response to the change in reactor load.
2. Description of the Prior Art
There are several factors which are theoretically associated closely with the control of the output of a boiling-water reactor. Of those factors, the method of adjusting the position of control rod and the method of controlling the recirculating flow are preferably in practical application. The reactor has several tens of control rods inserted in the reactor core, each control rod being made of, for example, boron which is a substance having a property of absorbing neutrons and each control rod being surrounded by several fuel rods. The fuel rods or slugs generate heat due to nuclear fission and the quantity of heat to be generated will be changed by the change in the number of neutrons which cause the following chain-reaction. The control rod serves to absorb the neutrons for the purpose of controlling the heat generation in the reactor. The deeper are the control rods inserted into the core of the reactor, the greater is the number of the absorbed neutrons and therefore the smaller is the quantity of heat generated. On the other hand, the farther are they withdrawn from the core of the reactor, the smaller is the number of the absorbed neutrons and the greater is the quantity of generated heat. The feature of this output control by adjusting the position of control rods is that the quantity of heat generated only in the neighboring fuel rods is changed but that in the remote fuel rods cannot be controlled. Thus, the region of control by each control rod is local so that local power range monitors are provided in the vicinity of the fuel rods. In a typical example, four fuel rods are arranged about a single control rod and four local power range monitors are disposed in the vicinity of one of the four fuel rods, along the axial direction thereof. The local power range monitors detect the neutron flux. A plurality of blocks, each consisting of a control rod, four fuel rods and four local power range monitors, constitute the core of a reactor. The core is located in the center of the reactor, usually immersed in water which is used as a coolant and moderator. Another practical method of controlling the output of the reactor is to control the recirculating flow of coolant. According to the method, for example, the water in the upper part of the reactor is drawn out, pressurized by the recirculation pump and recirculated from the lower to upper portion of the reactor, with the velocity of the flow of the recirculating water controlled. In this case, the amount of void (foam) created in the reactor depends on the temperature of the void generating surface: the greater is the recirculating flow, that is, the higher is the effect of cooling, the smaller is the amount of generated void. The void has a negative reactivity so that the amount of void generated in the reactor affects the output of the reactor. Namely, if the recirculating flow is increased to promote cooling efficiency, the void is decreased and the reactor output is also increased, and if the recirculating flow is decreased to reduce the cooling capacity, the void is increased to decrease the reactor output. The feature of the control of recirculating flow is the uniform distribution of heat generation in the direction of radius for the reactor core. It is therefore considered that the change in the reactor output due to the adjustment of the control rod is local while the change due to the control of the recirculating flow is uniformly spread over the core.
As described above, there are two practical method of controlling the output of the boiling-water reactor and they contribute differently to the reactor output so that they are usually used in combination. Namely, the local control by the control rod alters the steady state distribution of the output in the reactor core and may sometimes damage some particular fuel rods due to overheating caused when the rod is withdrawn while the general control by controlling the recirculating flow is restricted by the ratings of the recirculation pump, i.e. the maximum and minimum rpm's of the pump. For these reasons, the operation to withdraw the control rods when the reactor is at the high output performance is prohibited. The operation to withdraw the control rods is permitted only when the reactor is delivering less than about 50% of the rated output. In an operating range where the reactor is delivering more than about 50% of the rated output, the control of recirculating flow is exclusively employed.
The conventional atomic power plants are run with their loads fixed constant in view of safety of the reactor. However, it will be necessary in the near future to run several atomic power plants under variable load following since the proportion of the electric power generated by atomic power plants to that generated by other system will have been increased due to the future increase in the number of atomic power plane installations. It will be able to operate under variable load following since the safety and reliability of the reactor will have been improved through the experience obtained due to years of running. The most important thing in this case of variable load performance is the relationship between the reactor output and the coolant flow through the core of the reactor (approximately proportional to the recirculating flow), that is, it is necessary that the coolant flow through the reactor core should be sufficient to match the corresponding reactor output, irrespective of whether the output is changed by control rods or recirculating flow of coolant, so as not to overheat the core assembly and its associated members.
As for the control rods, a means for preventing the local overheat due to the withdrawal of the control rod has been proposed. This means is termed "rod block monitor" and the rod block monitor compares the recirculating flow with the outputs of the local power range monitors in the vicinity of the control rod to be withdrawn and when the outputs are greater than the maximum allowable output determined by the recirculating flow, the withdrawal of the particular control rod is prevented. In other words, the control rods can be withdrawn only when the reactor core is sufficiently cooled.
As regards the control of recirculating flow, no means for preventing the overheat of the members in the reactor has not yet been taken into consideration. It has been proved, however, that such a means is essential even in the case of the recirculating flow control if the reactor output is changed to a great extent. During the course of operation of a nuclear reactor, the fission fragments and their many decay products are produced and accumulated. These products are normally generated in proportion to the reactor output and have a property of absorbing neutrons. However, they provide a transient, counter reaction to the reactor output and when the reactor output is stepwise increased (decreased), the fission products temporarily decrease (increase) and thereafter increase (decrease) until the rate of generating the products has been saturated for the renewed reactor output. It takes several hours for this response to be completed and the predetermined load demand cannot be met unless the effect of the fission products as well as of the control rods and the recirculating flow is taken into account during the transient. Namely, the increase in the load, for example in a stepwise manner, can be followed up in a few minutes by increasing the recirculating flow, but in this case the change in the reactor load cannot be correctly followed unless the effect of the reactivity by the fission products taking place rather slowly for several hours is compensated. In such a case, the products decrease over 2 to 3 hours and the reactor output gradually increases though the positions of the control rods and the recirculating flow are both fixed. If the reactor is left uncontrolled, the members in the reactor core are overheated since the recirculating flow is fixed. In order to prevent the overheat, the control rods must be further inserted or the recirculating flow must be decreased. Such a danger of overheat usually takes place at high output performance so that the recirculating flow is decreased instead of further inserting the control rods, to maintain the reactor output constant. The decrease in the recirculating flow is continued until the products have begun to increase. After the products have begun to increase, the reactor output gradually decreases though the positions of the control rods and the recirculating flow are both fixed. In order to compensate for the decrease in the reactor output, the recirculating flow is again increased. These complicated operations are required whenever the reactor output is largely changed. Nothing has been described of the operations to diminish the reactor output, but it is a matter of course that similar complicated operations are needed to compensate for the influence by the fission products. Among these series of recirculating flow controls, it is when the reactor output is to be kept constant by decreasing the recirculating flow that the overheat of the members in the reactor must be prevented. Under such circumstances, since the recirculating flow is decreased while the reactor output is constant, then there is a danger of causing overheat in the members of the reactor due to the deficiency of recirculating coolant flow. As described above, when the reactor output is changed, there is needed a means for preventing the overheat of the members of the reactor, not only for the adjustment of the control rods but also for the recirculating flow control.